Transformers isolate an output from an input. The side that is connected with the input is designated as the primary side, and the side that is connected with the output is designated as the secondary side.
To increase power density and efficiency in supplying power from the input to the output, semiconductor switches such as Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) and Insulated Gate Bipolar Translators (IGBTs) are employed as synchronous rectifiers. Specifically, the semiconductor switches are able to replace rectifying diodes, because the semiconductor switches have a tiny conduction resistance when they are driven “ON.” Compared with the diodes, the semiconductor synchronous rectifiers can increase efficiency significantly and, further, reduce heat dissipation.